1. Field of the Invention
This invention relates to leak testing systems, and more particularly to a method and apparatus for electronically performing dynamic balance leak tests wherein a part is filled, evacuated, or surrounded, with a test medium at a predetermined pressure or vacuum, and the change of that pressure or vacuum over a period of time, compared with the reference pressure, is measured. The actual testing sequence consists of filling, evacuating from, or surrounding the part to be tested with air, or another testing medium, bringing it to a predetermined test pressure or vacuum, disconnecting or shutting off the test part from the source of testing medium, allowing a short time for the system to "balance" itself, and thereby eliminate parasitic influences such as changes in barometric pressure, variations in the air supply systems, adiabatic heat, valve operation effects, etc., and then measuring the differential pressure after the balance time to obtain an accurate initial differential pressure reading. We call such a reading a "live zero" reading because it is not the same for each test, such as a predetermined pressure or vacuum reading would be, but instead is one that can change for every test to reflect actual conditions within the test system.
The differential pressure is again measured at the end of a predetermined test period. It should be understood that the differential pressure readings are taken at the test part with respect to the source.
The difference between the differential pressure at the beginning of the test period, and at the end thereof, constitutes the "change in pressure", which is the measure of leakage of the part. Leakage may be an indication of faulty casting, faulty machining, improper materials, or other similar problems, and such leakage may call for rejecting the part if it is above permissible limits.
It should be understood that the amount of permissible leakage is a standard set by the manufacturer on the basis of experience and the demands which the part must satisfy. Setting such a standard may be preceded by experimentation, operating on the basis of the tentatively set standard, analyzing field reports, etc.
2. Description of the Prior Art
Applicants' assignee for many years has performed dynamic balance leak tests for its customers utilizing a pneumatic apparatus described in U.S. Pat. Nos. 3,248,931 and 3,387,619, and the advantages of such system over the systems known in the art is fully explained in the specification of those patents, which are specifically incorporated herein by reference.
However, as good as such system is, and such system is better than anything available in the art at the present time, the best sensitivity which could be obtained with such a system was 0.01 inches of water. While with this sensitivity such systems are still satisfactory, and are serving well for many, many, testing purposes, as the demand for faster and faster and more sensitive leak testing systems become apparent, Applicants were faced with the necessity of developing an even faster and more sensitive leak testing system.
Some attempts were made to make the pneumatic system more sensitive, but it was immediately found that the mass and the bulk of the parts used therein made any prospects for greatly improved sensitivity very small, and Applicants then decided to try an electronic approach to leak testing in an attempt to get better sensitivity, because such approach was becoming more and more practicable with the advances in the electronics' art.
Applicants' first attempt at providing an electronic system was to mimick the pneumatic circuitry of the aforementioned leak test system with electronic analog circuitry. However, because it was preferable to have a drift-free type of circuitry, it was desirable to have the calculations performed on a digital basis. This led to the use of calculator integrated circuit chips to perform the digital calculations and comparisons. However, this attempt was not successful because Applicants quickly ran into major problems trying to interface the calculator integrated circuit chips required with other parts of their system.
At about the same time this was occurring, the electronics field was experiencing the advent of microprocessors and microcomputers utilizing microprocessors. Knowing of this, the Applicants decided to try and use a microcomputer in their attempt to arrive at a satisfactory leak testing system. After consideration of the above-mentioned problems, it was thought that use of a microcomputer system in a dynamic balance leak test system would be most satisfactory. After much effort, this approach proved to be correct, and Applicants arrived at the system disclosed, and claimed, herein.
By taking this approach, Applicants reduced or eliminated many of the sensitivity problems in present day electronic leak test systems as discussed below.
The typical electronic leak testing system in the art today decreases in absolute sensitivity as the pressure at which you perform the test is increased. In one presently available electronic test system not using dynamic balance leak test it is common to have two meters, a percent pressure meter graduated from 0% to 100% with 50 graduations, such that the reading 100% equals the full scale pressure of the transducer, which is greater than the pressure at which the test is being performed. A second meter designates pressure change, that being a zero center scale with 20 graduations on the left-hand side from -1.0 to 0 and with 20 graduations on the right-hand side from 0 to +1.0.
This available test system has a sensitivity selection of 0.1% to 10% of full scale, and when using the finest sensitivity range for example, if the 100% pressure (test pressure) is 20 lbs. per sq. in. (p.s.i.) gauge, the sensitivity is 20 lbs. per sq. in. multiplied by the sensitivity (0.1%=0.001) divided by the number of graduations on one half of the pressure change scale (20). Thus, the smallest change you can read is (20.times.0.001)/20=0.001 p.s.i., which equals 0.03 inches of water.
When one tests at 100 p.s.i., which would now be the 100% pressure reading, multiplied by 0.001 and divided by 20, this equals 0.005 p.s.i., or 0.15 inches of water, as the smallest sensitivity which can be read. By using dynamic balance, Applicants have eliminated this change in absolute sensitivity.
Further, with this type of division of the analog electrical signal into actually 20,000 analog parts in the immediately previous example, you have such a small signal that electrical noise becomes a serious problem. If the transducer used provides a 10-volt analog signal, which is divided into 20,000 analog parts of 0.5 millivolts, each analog part can be of a smaller value than much of the electrical noise commonly encountered, thereby reducing the accuracy of the reading.
Another inconvenience with present day electronic leak test systems concerns the need to change to different transducers for different test pressures. Typically, if one is going to test at 20 p.s.i., one will use a pressure transducer rated at 20 p.s.i. to get the best sensitivity possible (i.e. one wouldn't normally use a 100 p.s.i. transsducer at 20 p.s.i.). However, if the system is to be used for many different tests at many different pressures, frequent changing of transducers will be necessary if the best sensitivity is to be maintained.
Another problem the Applicants were aware of in their search for a better system is the fact that in a pressure decay leak test system, which is the type of system which has just been discussed, even the point of 0.1% sensitivity is largely unattainable because changes in the operating conditions and operation of the valves will cause changes in the transducer reading of a magnitude equal in some cases to the magnitude of the signal representing the sensitivity reading.